The entry of insulin, epidermal growth factor, triiodothyronine and alpha2 macroglobulin into cells has been studied by video intensification microscopy and immunocytochemistry. Hormones initially bind to diffusely distributed receptors. The hormone ligand complexes then cluster in coated pits and are internalized in a specialized vesicle termed a receptosome. Entry is blocked by inhibitors of transglutaminase suggesting cellular TG is involved in the process.